U.S. Pat. No. 4,516,840 discloses an arrangement for determining the in-focus condition of a stereomicroscope. In this arrangement, the image of an illuminated marking is projected on an object via one of the two viewing beam paths. In the second viewing beam path, the beam reflected from the object is coupled out via a corresponding divider mirror and deflected onto a photoelectric transducer element. This photoelectric transducer element generates a control signal for the motorized focusing in dependence upon the position of the registered reflecting beam.
The imaging of a marking via the viewing optics is disadvantageous in an arrangement of this kind. A diaphragm of any desired form is imaged on the object with the lateral dimensions of the imaged marking being greater than the resolving capability of the objective. This situation gives rise to difficulties if the surface of the viewed object has regions of an intensely different reflectivity. This is, for example, often the case in surgical microscopes. Here, the marking reflected or projected onto the photoelectric transducer element has such a reflection structure under certain circumstances. Accordingly, difficulties result during the reliable evaluation of the position of the reflected marking on the photoelectric transducer element.
If, in contrast, a marking is imaged which is at least in one dimension less than the resolving capability of the objective, then no reflection structure is registered in the image of the reflected beam on the photoelectric transducer element; however, when imaging, only a marking of very weak intensity is imaged on the object.
Accordingly, if it is desired to equip a surgical microscope having a brightly illuminated viewing field with such a device, then significant problems occur for the detection of the projected marking since the contrast differences on the brightly illuminated object are slight. The use of a laser as the imaging light source has, in addition, the consequence that for an expanded light marking on the object, so-called speckles or uncontrolled interference phenomena on the photoelectric transducer element make the evaluation as difficult as the reflection phenomena described above. If for this reason, a correspondingly narrow diaphragm is imaged, then here too intensity problems on the brightly illuminated object result as described above.